Conventionally, an internal combustion engine; for example, an automobile engine, uses an ignition plug for igniting an air-fuel mixture by means of spark discharge (which may be referred to merely as “discharge”). In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, use of a plasma jet ignition plug is known, since the plasma jet ignition plug provides quick propagation of combustion and exhibits such a high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Such a plasma jet ignition plug has a structure in which an insulator (housing) formed of ceramics or the like surrounds a spark discharge gap between a center electrode and a ground electrode (external electrode), thereby forming a small-volume discharge space called a cavity (chamber). Taking as an example a plasma jet ignition plug which uses a superposition-type power source, in igniting an air-fuel mixture, a high voltage is first applied between the center electrode and the ground electrode so as to perform spark discharge. By virtue of associated occurrence of dielectric breakdown, current can flow therebetween at a relatively low voltage. Thus, through transition of a discharge state effected by a further supply of energy, plasma is generated within the cavity. The generated plasma is emitted through a communication hole (external-electrode hole) which is formed through the ground electrode, thereby igniting the air-fuel mixture (for example, see Japanese Patent Application Laid-Open (kokai) No. 2006-294257, hereinafter referred to as Patent Document 1).
According to Patent Document 1, a ground electrode is formed integrally with a metallic shell which has threads for mounting the plasma jet ignition plug to an engine. However, according to another disclosure, a ground electrode having a communication hole and a metallic shell are formed as separate members (for example, see Japanese Patent Application Laid-Open (kokai) No. 2007-287665, hereinafter referred to as Patent Document 2).
Meanwhile, in order to ensure resistance to spark-induced erosion of the ground electrode, which performs spark discharge, the ground electrode has high thermal conductivity and has such a structure as to readily release heat to the engine through the metallic shell. If plasma emitted from the cavity comes into contact with the wall surface of the communication hole of such a ground electrode, heat is transferred from the plasma to the ground electrode, so that energy of the plasma is apt to be removed. In the plasma jet ignition plugs described in Patent Documents 1 and 2, there is not much difference in diameter between the communication hole and the cavity. Thus, in emission of plasma, the plasma is apt to come into contact with the ground electrode. In order to reduce loss of energy caused by contact between plasma and the ground electrode, expansion of the opening diameter of the communication hole is preferred for rendering an emitted plasma unlikely to contact the ground electrode (for example, see Japanese Patent Application Laid-Open (kokai) No. 2007-287666, hereinafter referred to as Patent Document 3).
In the plasma jet ignition plugs described in Patent Documents 1 to 3, plasma is emitted through the communication hole of the ground electrode, and the ground electrode is formed annularly. If the ground electrode assumes a form employed in general spark plugs (e.g., the form of a bar), the volume of a portion of the ground electrode which comes into contact with plasma emitted from the cavity can be reduced, whereby loss of energy of plasma can be inhibited (for example, see Japanese Patent Application Laid-Open (kokai) No. 2000-331771, hereinafter referred to as Patent Document 4).
Problems to be Solved by the Invention
However, as in the case of Patent Document 3, the greater the opening diameter of the communication hole of the ground electrode, the more likely the front end face of an insulator is to be located on a path of spark discharge between a center electrode and the ground electrode. Accordingly, spark discharge creeps on the front end face of the insulator; i.e., spark discharge is performed in the form of so-called creeping discharge. The creeping discharge is apt to erode a portion of the surface of the insulator located on the path of spark discharge; i.e., so-called channeling is apt to arise. At this time, the path of spark discharge extends from the front end face of the insulator, passes an opening end of the cavity, and extends toward the center electrode located within the cavity. Thus, an edge portion of the opening end is apt to be eroded. Accordingly, a path of spark discharge which passes through the eroded portion becomes shorter in distance than other paths. Therefore, creeping discharge becomes more likely to arise along the path of spark discharge, resulting in a risk of local channeling progressing.
According to Patent Document 4, since the intermediate electrode provided between the center electrode and the ground electrode is electrically conductive, spark discharge arises between the ground electrode and the intermediate electrode. In this case, electric fields are apt to concentrate at an opening end, which assumes the form of a sharp edge, of the intermediate electrode. Thus, the opening end is apt to become a starting point of spark discharge. Furthermore, since the ground electrode is in the form of a bar, spark discharge concentrates at a single circumferential position on the opening end. Accordingly, the opening end of the intermediate electrode is apt to be eroded at a specific position, resulting in risk of local channeling occurring. Also, according to Patent Document 4, the distal end of the ground electrode is located in such a manner as to greatly interrupt emission of plasma; i.e., removal of heat by the ground electrode (loss of plasma energy) is not sufficiently considered.
The present invention has been achieved for solving the above-mentioned problems, and an object of the invention is to provide a plasma jet ignition plug capable of inhibiting the occurrence of channeling while reducing the removal of energy of plasma through a ground electrode at the time of plasma emission.